Growing environmental awareness has developed a market need for recycling plastic items. Such items are made from nonrenewable petrochemical resources, consume diminishing landfill space, and decompose very slowly. The market for recycled plastic is cost sensitive, and its ultimate size, success, and profitability result from the degree to which automated systems can sort a wide variety of plastic items and, in particular, plastic containers such as beverage bottles. Plastic container sorting has value because containers consume an inordinate portion of landfill volume.
Systems and methods are already known for sorting plastic items by size, color, and composition. In particular, U.S. Pat. No. 5,150,307 for a COMPUTER-CONTROLLED SYSTEM AND METHOD FOR SORTING PLASTIC ITEMS describes a sorting system in which baled plastic items, including containers, are broken apart into pieces, singulated on a split belt, and spun to lengthwise orient them for inspection by a length-detecting photocell array and an RGB color reflectance imaging camera. When the length is known, the center of each item is estimated so that most background data can be eliminated from the RGB reflectance image to speed up a time-consuming composition and color analysis. Reflectance images are subject to color contamination by labels, so the system performs an image grid analysis by which an image edge is located and the dominant RGB color is determined for each grid element located along the image edge. The item edge is assumed to include a minimum of color-contaminating label data. Item discharge utilizes a discharge conveyor position-synchronizing rotopulse, an item-indicating photoeye, an item-sorting mechanical distribution gate, and an item-discharging air ejector.
Such a sorting system is costly, overly complex, and prone to unreliability. Spinning items to achieve the lengthwise orientation increases the probability that adjacent items can be knocked into misalignment. Length and center determination, coupled with background elimination, edge determination, and grid analysis, is an overly time-consuming color analysis method that is subject to edge-induced color errors. Moreover, using only a reflectance image does not provide for optimal analysis of transparent and translucent items. Finally, because they are light and have a variety of shapes and sizes, plastic containers tend to float, roll, and shift position easily on a conveyor belt. Even though the above-described sorting system can handle plastic containers, it is needlessly complex, potentially unreliable, and therefore not optimally cost effective for sorting plastic containers.
U.S. Pat. No. 5,141,110 for a METHOD FOR SORTING PLASTIC ARTICLES describes using polarized light and crossed linear polarizers to classify the composition of transparent or translucent plastic articles as either polyethylene terephthalate ("PET") or polyvinyl chloride ("PVC"). Color analysis entails using an unacceptably slow mechanically positioned color filter technique. Opaque plastic articles are inspected with scattered and/or refracted X-rays, a known hazardous technique. The patent does not describe how color analysis is accomplished for opaque articles. In any event, proper inspection is said to require delabeling or otherwise avoiding labels, but no way of avoiding labels is described.
U.S. Pat. No. 4,919,534 for SENSING OF MATERIAL OF CONSTRUCTION AND COLOR OF CONTAINERS describes using two wavelengths of polarized light to determine the composition and color of transparent and translucent containers. In particular, determining the composition as glass or PET and further determining color entails calculating a difference in the transmitted intensity for polarized light at each of the two wavelengths, normalizing by the sum of the transmitted intensities, and using the normalized difference as a color index for characterizing the color of the container. Labels are considered opaque and are, therefore, ignored. Opaque containers cannot be analyzed by this technique.
U.S. Pat. No. 5,085,325 for a COLOR SORTING SYSTEM AND METHOD, assigned to the assignee of this application, describes using line-scanning cameras to sort moving articles on the basis of reflected RGB colors of visible light. Colorimetric accuracy is ensured by normalizing the light sensitivity of each camera sensor element, digitizing each RGB pixel value, and using the digitized value as an address into a color lookup table ("CLUT") that stores predetermined accept/reject data. The CLUT address is an 18-bit word formed by concatenating together the six most significant bits of each R, G, and B normalized and digitized color data. Such color data are said to be in a three-dimensional color space. CLUT output data can be size classified by a filter lookup table ("FLUT") and/or image processed in an image memory. Statistical- and histogram-based methods for loading the CLUT and FLUT with accept/reject and filtering data are also described. This system is primarily used to detect spot defects, such as eyes, in opaque articles, such as potato strips.
What is needed, therefore, is a simple, cost-effective plastic container sorter that is capable of accurately classifying labeled containers of any size, opacity, transparency, color, or orientation. Moreover, plastic containers are positionally unpredictable because their shape and light weight allows them to slip, roll, and slide during inspection. Therefore a simple and reliable system is needed for tracking and ejecting the classified containers.